GNIRS Detector Characteristics and Read Modes

The properties of the Aladdin III InSb detector array for the GNIRS science channel are given in Table 1 below. The array has nicely uniform response, only a few regions of bad and dead pixels (see table below) very low dark current and low read noise in the lowest background mode. The bias voltage may be adjusted to increase the well depth for thermal
IR (L and M band) observations or for very bright targets. InSb detectors have nearly 100 percent internal quantum efficiency (QE) from 0.4 to 5 microns, with a rolloff near the cutoff wavelength of 5.5 microns. The surface of the material is highly reflective (36 percent), but this is reduced by an antireflection coating that gives the array an effective QE of ~90 percent over the 0.9-5 micron interval.

There is a tradeoff between the detector read noise and the time required to read out the array. The best choice depends on the brightness of the object and the background at a given wavelength (see the Observing Strategies page for more guidance). The four read modes which can be used for GNIRS observations, and the read noise and exposure times for each are given in Table 2.

Table 2: Read Modes

Read Mode

Low Noise Reads

Digital Averages

Read Noise(e-)

Overhead/ReadandMinimum exp. time* (sec)

Recommendedmin. exp time* (sec)

RecommendedUse

Very Bright Objects;high background

1

1

155

0.2

<1.0

low res. M

Bright Objectsmedium background

1

16

30

0.6

~2.0

Exp ~1 - 20sand/orsqrt(sig+bkgnd)>80 e/pix

Faint Objectslow background

16

16

10

9.0

20.0

Exp ~20 - 60 sand/orsqrt(sig+bkgnd) < 50 e/pix

Very Faint Objectsvery low background/

32

16

7

20.0

60.0

Exp > 60 sand/orsqrt(sig+bkgnd)< 20 e/pix

* The minimum exposure times are
the times required to read the array. The recommended minimum exposure
times allow efficient observing (spending at least 2/3 of the time
actually exposing the array as opposed to reading it).

Some problems and features related to the detector and its controller are described on the "Known Issues" page.